Humanity evolved to cope with 30°C+ heat, says prof

Scorchio conditions made us hairless, upright, sweaty

Many of humanity's distinctive features - walking upright, hairlessness, the ability to sweat copiously - arose due to the fact that the place where we evolved has been scorchingly hot for millions of years, according to noted boffins.

The cradle of humanity, according to most research, was the Turkana Basin in Kenya's Great Rift Valley. Today this is a terrifically hot and arid place, but some scientists have argued that during the Pliocene and Pleistocene eras, when humanity was making its first appearance in the area, it must have been cooler and/or more wooded.

But now a crew of researchers headed up by Benjamin Passey of Johns Hopkins University say this isn't so - the Turkana area has always been cruelly hot, generally above 30°C and sometimes above 35°C, for the whole time humanity has existed.

“The ‘take home’ message of our study,” says Passey, “is that this region, which is one of the key places where fossils have been found documenting human evolution, has been a really hot place for a really long time, even during the period between three million years ago and now when the ice ages began and the global climate became cooler.”

The prof and his colleagues have determined this by examining rare isotopes of carbon and oxygen bonded together in soil carbonates. According to their calculations, at the time when humanity was appearing the daytime air temperature was generally 30°C or even higher.

“Thus, we can say that the ‘thermal hypothesis’ is credible," argues Passey.

The thermal hypothesis says that humans began walking upright, became unusually sweaty (for the animal kingdom) and shed their body hair in order to stay cool, rather than for other reasons such as shifting habitats among local wildlife. According to this theory, supported by the new isotope data, humanity is actually designed to live in a scorchingly hot climate. The human race's subsequent expansion across the globe has required innovations such as clothing, use of external energy sources (fire) etc.

Passey and his colleagues' research is published in the Proceedings of the National Academy of Sciences. ®